1. Field of the Invention
The invention relates generally to catalytic coating compositions and to the method by which they can be applied to the surface of solid structures. In particular, the invention is directed to catalytic compositions for use on the walls of cooking devices to effect oxidative decomposition of residues which are deposited thereon during various cooking operations.
2. Prior Art
One of the less pleasant aspects of cooking is the cleaning-up afterward. In particular, the cleaning of household ovens has long been one of the less elegant chores in the kitchen of any household. It is, of course, well-known that the surfaces of ovens and other cooking devices become coated with residue from grease and food particles.
There are three basic approaches to the problem of dirty oven deposits: (1) ignore them; (2) remove them periodically; or (3) prevent them. Though the first of these has frequently been the most prevalent approach, the latter approaches have gained in popularity as they have become less onerous to carry out.
For the last several years, the most prevalent approach to the removal of household oven deposits has been the use of dilute alkali solutions which are sprayed on the surface, allowed to stand for several hours and then removed with a wet sponge. While this method is effective, it is messy, it necessitates the handling of hazardous alkali solutions and is time-consuming. Simultaneously, many oven manufacturers approaches the periodic removal problem by making ovens capable of attaining extremely high temperatures by which the deposits could be removed by burning them off. Though this method is likewise effective, it consumes large amounts of energy and seriously reduces the margin of fire safety in many oven installations.
More recently, domestic appliance manufacturers have approached the problem by applying a coating of catalyst to the interior walls of ovens by which the pyrolytic temperature of oven spatterings is reduced. Thus, deposits on oven walls are removed almost as soon as they are formed at normal cooking temperatures without the use of either extremely high temperatures or alkali solutions.
In this regard, U.S. Pat. No. 3,266,477 to Stiles discloses the use of oxidation catalysts on cooking surfaces to remove food and other cooking residues by catalytic oxidation. Such ovens are disclosed to be self-cleaning at temperatures of 204.degree. to 260.degree. C. In U.S. Pat. No. 3,271,322 to Stiles, there is disclosed a catalytic surface in which catalytic metals are supported on a particulate carrier and the particles anchored to a coating of polytetrafluoroethylene. A further advance in the art was disclosed by Stiles and Yates in U.S. Pat. No. 3,460,523 wherein porous catalytic coatings are provided by the use of catalytic metal oxides bound to the surface with alkali metal silicate binder. In copending U.S. Patent Application Ser. No. 553,425, filed Feb. 26, 1975, (U.S. Pat. No. 3,993,597) Stiles presents a still further advance in the art of catalytic oven coatings with the use of mixtures of the corresponding hydroxides or carbonates with catalytic metal oxides in coating systems using an alkali metal silicate binder. It is also disclosed that, up to a limit of 5.0% by weight, humectant can be used in such coatings to prevent crazing of the surface. Such humectants include glycerin, ethylene glycol, diethylene glycol and the like.
Though self-cleaning ovens using such catalysts are typical of the current production of domestic cooking devices, such convenience has not heretofore been available to owners of the estimated 50 million household ovens in use in the U.S. which were manufactured without catalytic coatings. Thus, there is a real need for catalytic coating compositions which can be applied to existing uncoated ovens by members of the household.
It is, of course, not only desirable that such coatings can be applied at home, but also that they can be removed in the event they are rendered less active by use over a long period of time.